Because most pathogens colonize and invade the host at mucosal surfaces, the induction of immunity at these sites is a rational and attractive approach to prevent infection (1). Mucosal routes for vaccine delivery are non-invasive, so administration is relatively simple and inexpensive. Furthermore, the potential to induce a range of mucosal and systemic immune responses after mucosal vaccine delivery allows the possibility of effective immunization against many diseases. For example, specific IgA alone can protect mice against intranasal infection with influenza (2) and intestinal infection with Vibrio cholerae (3). However, mucosal delivery of nonreplicating immunogens typically does not stimulate strong immune responses. Where responses are induced, the delivery of multiple high doses is often necessary (4). In addition, mucosal delivery of immunogens frequently results in systemic unresponsiveness (1).
A number of strategies may be used to enhance responses to mucosally delivered vaccines. Live bacterial and viral vectors which colonize the mucosae can be used to deliver immunogens (5). Imparting particulate characteristics to immunogens by association with biodegradable microparticles (6) or liposomes (7) can also enhance mucosal immunogenicity.
Another approach is the use of lectin-like molecules with adjuvant properties. The most powerful mucosal adjuvants identified to date are cholera toxin produced by Vibrio cholerae (CT) and heat-labile enterotoxin (LT) from enterotoxigenic strains of Escherichia coli (8, 9). CT and LT are well-characterized mucosal immunogens and adjuvants for bystander proteins. These toxins contain separate A and B subunits (referred to as CTA and CTB, respectively). The B subunits mediate binding to cell surface receptors (20). GM1 ganglioside is considered to the principal receptor for CT (21), but CTB may bind to cell surface receptors other than GM1 (22). After binding of the B subunit, the A subunit reaches the cytosol and activates adenyl cyclase leading to a large increase in [cAMP]i (10, 11). LT is structurally and functionally similar to CT and is comparable to CT as a systemic or mucosal adjuvant (23, 24). In mice, CT strongly stimulates humoral and cell-mediated immune responses, including mucosal IgA production and cytotoxic T cell effector functions (10). Stimulation of toxin-specific local and systemic responses and responses to co-administered immunogens distinguish these molecules from most soluble proteins which are poorly immunogenic when administered mucosally (10, 11). The toxicity of these molecules, however, prevents clinical application.
Certain plant lectins have been investigated as agents for specific targeting of molecules to a mucosal epithelium. Plant lectins are proteins containing at least one non-catalytic domain, which binds specifically and reversibly to a monosaccharide or oligosaccharide (13). For example, Giannasca et al. (14) discloses that intranasal immunization with a lectin-immunogen conjugate stimulated induction of specific IgG antibodies, while immunogen alone or admixed with lectin did not. U.S. Pat. No. 4,470,967 discloses that a complex of a glycoprotein immunogen with a lectin can act as an adjuvant to increase the immune response against the immunogen. Similarly, WO 86/06635 discloses a chemically modified immunogen-lectin complex which can be used to elicit an immune response in vertebrates, including mammals. In each of these cases, however, the lectin was physically coupled to the immunogen. This requires at least one extra preparation step and may actually alter an epitope of the immunogen against which an immune response is desired, such as an epitope against which a neutralizing immune could be directed.
Thus, there is a need in the art for simple, effective, and non-toxic methods of increasing immune responses in a mammal, particularly after mucosal administration, without the need to complex the immunogen with another molecule and potentially mask or alter desirable epitopes.